Low temperature merchandisers for frozen foods are designed to maintain product temperatures in the display area less than about 0° C., and more particularly, less than or equal to about −15° C. for frozen food and below about −24° C. for ice cream, which in the past have required cooling coil temperatures in the range of about −24° C. down to about −37° C. Low temperature merchandisers are generally kept at temperatures less than about 0° C., and more particularly below −21° C. Medium temperature merchandisers maintain non-frozen food items, at temperatures generally in the range of about −1° C. to about 5° C.
Multi-shelf reach-in merchandisers for the storage and display of fresh and frozen food products (including ice cream) provide a generally vertical display of the product for greater visibility and product accessibility to shoppers. In order to prevent the escape of cold air into the shopping arena, a transparent glass or plastic front door typically closes the display area of the merchandisers. Glass and plastic are poor thermal insulators. As a result, the doors are conventionally formed by two or three spaced apart panes of glass, defining one or two air spaces to increase the thermal insulation of the door. The air spaces must be sealed for maximum insulating effect, and to prevent entry of moisture into these air spaces. Moisture in the air space condenses on the cold glass (or fogs) and obscures viewing of the product in the merchandiser. In the past, sealing of the air space has been accomplished by forming an “insulating glass unit” or “IG unit” (sometimes called a “glass pack”) which consists of opposing glass panes (called “lights” or “lites”) separated by a metallic spacer secured by a suitable polymer (e.g., polysulfide, polyisobutylene, etc.). The glass pack is placed in a metal frame to complete the door. Thus, the door assembly process usually has involved two separate steps of forming the sealed air spacers because it has a good strength-to-weight ratio. In addition, metal is an excellent moisture barrier and when used as a spacer seals the air space from moisture for many years. However, metal has two important drawbacks when used in reach-in-doors. The first is that metal is a poor thermal insulator, and the second is that metal is an excellent electrical conductor.
Conventional attempts to attenuate thermal conduction through the metal in the door generally involve placing barriers in the path of thermal conduction. Others have attempted to partially or entirely replace the metal frame with a polymeric material having a substantially lower thermal conductivity. However, it will be noted that these attempts to reduce the metal used in the doors have not eliminated the metallic spacers, nor have they replaced the need for sealing glass lites before forming the frame.
The electrical conductivity of metal has also been a hindrance because in the past electrical power was commonly used to heat one or more surfaces of the glass lites in the door in order to prevent condensation from collecting and obscuring vision through the glass or plastic panes. For instance, the moisture in the relatively warm ambient air of the store readily condenses on the outside of the door if it was not heated. Also, when the door is opened, moisture condenses on the cold inside glass surface. Without heating, this condensation would not clear quickly and so the view of the product in the merchandiser would be obscured. Typically, two types of heaters have been used: (1) an anti-sweat heater wire is applied to the perimeter of the metal frame; and (2) a semi-conductive coating or film (e.g., fluorine-doped tin-oxide) on the inner surface of the outer glass lite in the door is powered by bus bars along opposing edges of the lite to provide an electrical potential causing a current to flow through the electrically-conductive film and produce heat. It has been necessary to keep wiring and bus bars supplying the electric power carefully insulated and isolated from the outer metal door frame and the inner metal spacer. This means that a portion of the heating film had to be eliminated at the edge margin where there would be contact with metal. Avoiding electrical wiring and heating is desired.
Therefore, new ways are sought of preventing or inhibiting fogging of glass or plastic substrates when a door is exposed to a cool environment (as discussed above and in more detail below), and is then exposed to moist air ambient conditions upon being opened. The cool inside surface of a refrigerator door may be exposed to an ambient environment for a few seconds, thirty seconds, or longer, depending on how long the customers or employees keep the door open. In other words, new ways to optimize visibility for the marketing of frozen food products are sought.